Japanese patent application Laid-Open No. 50-46791 discloses a photosetting composition comprising, as photosetting base polymers, a modified epoxy resin which is an esterification reaction product of an epoxy resin with acrylic or methacrylic acid and an oligoester diacrylate which is a coesterification reaction product of a polycarboxylic acid with a polyhydric alcohol compound and acrylic or methacrylic acid.
The above-mentioned type of photosetting composition is useful as a solvent-free type photosetting composition. However, the above-mentioned photosetting composition is disadvantageous in the following points.
That is, when the composition is photoset in ambient air by applying ultraviolet rays thereto, the rate of the photocuring reaction, which is represented by the so-called belt speed, is not satisfactorily high. Also, when a film of the photosetting composition formed on a tin-plated steel sheet or when an aluminium sheet is photocured by means of ultraviolet rays, the resultant photoset film exhibits poor bonding activity to the tin-plated steel sheet or aluminium sheet, poor impact strength, and poor impact bending strength. The photosetting composition therefore, cannot be practically used for industrially coating cans.
Japanese patent application Laid-Open No. 55-54315 (1980) discloses a photosetting composition which can produce a photoset resinous product having excellent heat-resistance and satisfactory flexibility. This type of photosetting composition contains, as photosetting base polymers, a modified epoxy resin which is a reaction product of an epoxy resin with acrylic or methacrylic acid and a thermosetting modified epoxy resin which is a reaction product of an epoxy resin with a thermosetting phenol-formaldehyde resin.
However, in the above-mentioned type of photosetting composition, the compatibility of the modified epoxy resin with the thermosetting modified epoxy resin is unsatisfactory. The photosetting composition is therefore, sometimes devitrified and becomes useless for practical uses. Also, when a film of the photosetting composition is photocured, the resultant film exhibits unsatisfactory impact bending strength. The photosetting composition is therefore, useless for coating cans.
Japanese patent application Laid-Open No. 56-41220 (1981) discloses a photosetting composition having a satisfactory photocuring reaction rate and bonding property and being capable of being converted to a photocured polymeric article having excellent impact strength. This composition comprises, as photosetting base polymers, a modified epoxy resin which is a reaction product of a liquid epoxy resin with a partial ester compound of an aromatic poly-carboxylic acid; and an oligoester diacrylate or dimethacrylate which is a reaction product of acrylic or methacrylic acid with an oligoester of an aromatic polycarboxylic acid with a polyhydric alcohol compound. This type of photosetting composition, however, is unsatisfactory in the poor impact bending strength of a photoset film of the composition.